1. Field of the Invention
The present invention relates to a backlight for illuminating a transmissive liquid crystal panel and to a liquid crystal display device having such a backlight. The present invention relates particularly to a backlight and a liquid crystal display device that permit the viewing of a displayed image only within a particular range of viewing angles.
2. Description of the Prior Art
A liquid crystal display device having a transmissive liquid crystal panel is so structured as to produce a desired image by illuminating the liquid crystal panel from behind with a backlight so that light is transmitted through particular pixels of the liquid crystal panel. This permits the viewing of a displayed image even, for example, at night when no ambient light is available.
A liquid crystal display device designed for use in a navigation system mounted on a car, an automated teller machine (ATM) installed at a banking institution, or the like is so configured as to permit the viewing of a displayed image only from a viewpoint within a particular range of viewing angles so that the displayed image cannot be viewed from a viewpoint outside that range of viewing angles. This is because, with a liquid crystal display device for car-mounted use, the range of angles of emergence of the light emerging therefrom is limited because, if an image displayed thereon is projected onto the windshield of the car, it may disturb the field of vision of the driver and lead to a car accident. On the other hand, with a liquid crystal display device for use in an ATM, the range of angles of emergence of the light emerging therefrom is limited so that no one can view the information displayed on the liquid crystal panel other than the one who is operating the ATM.
FIG. 26 shows the structure of a conventional liquid crystal display device that can control the range of angles of emergence of the light emerging therefrom. The liquid crystal display device 1 is composed essentially of a liquid crystal panel 6 and a backlight 7. The backlight 7 has a light guide plate 3 covered with a reflector sheet 4 made of polyethylene terephthalate (PET) foam. Along opposite sides of the light guide plate 3, light sources 2 are supported by a supporting member (not shown). The back surface 3b of the light guide plate 3 is formed as a non-glossy surface, so that the light emitted from the light sources 2 is incident on the light guide plate 3 and then emerges therefrom through its exit surface 3a as scattered light.
Above the light guide plate 3 are arranged a diffuser sheet 20 for producing diffused light and a light shield louver 5 for shielding the light incident within a predetermined range of angles. As shown in FIG. 27, the light shield louver 5 has light-transmitting layers 5a, which transmit light, and light-absorbing layers 5b, which absorb light, arranged, for example, at 50 xcexcm intervals and sandwiched between transparent base plates 5c. Thus, of the light incident on the light shield louver 5, the portion traveling within a range xcex8 of viewing angles is transmitted, but the portion traveling outside the range xcex8 of viewing angles is shielded.
Above the light shield louver 5, a transmissive liquid crystal panel 6 is disposed. The light traveling within the range xcex8 of viewing angles (see FIG. 27) and thus transmitted through the light shield louver 5 illuminates the liquid crystal panel 6, and is transmitted through particular pixels thereof so as to form an image. As a result, the image can be viewed from a viewpoint within a predetermined range of directions.
However, the conventional liquid crystal display device 1 described above, for example when it offers a range xcex8 of viewing angles of 90xc2x0, exhibits transmittance as shown in FIG. 29. In this figure, the angle of incidence (xc2x0) with respect to the light shield louver 5 is taken along the horizontal axis, and the transmittance (%) is taken along the vertical axis. As this figure shows, the transmittance is highest at an angle of incidence of 0xc2x0, and the transmittance linearly decreases as the absolute value of the angle of incidence increases.
Owing to the characteristics of the backlight 7 shown in FIG. 26, the light emerging from the light guide plate 3 exhibits a brightness distribution as shown in FIG. 28, offering the highest brightness around an angle of incidence of 0xc2x0 with respect to the light shield louver 5. As a result, the light transmitted through the light shield louver 5 and then incident on the liquid crystal panel 6 exhibits a brightness distribution as shown in FIG. 30.
In FIG. 28, the angle of incidence (xc2x0) with respect to the light shield louver 5 is taken along the horizontal axis, and the relative brightness (%) relative to the brightness (100%) at an angle of incidence of 0xc2x0 is taken along the vertical axis. In FIG. 30, the angle of emergence (xc2x0) with respect to the light shield louver 5 is taken along the horizontal axis, and the relative brightness (%) relative to the brightness (100%) at an angle of emergence of 0xc2x0 is taken along the vertical axis.
As FIG. 30 shows, the brightness is highest at an angle of emergence of 0xc2x0, and the brightness falls sharply as the absolute value of the angle of emergence increases. This is the reason that, as the viewpoint of the viewer varies according to his or her height, sitting height, or the like, the viewability of the liquid crystal display device 1 degrades markedly.
Moreover, in the production of the light shield louver 5, errors are inevitable in the intervals at which the light-transmitting layers Sa and the light-absorbing layers 5b are arranged. As a result, as shown in FIG. 31, when the liquid crystal display device 1 is viewed from the directions indicated by arrows P1 and P2, part of the viewer""s lines of sight are shielded by the light-absorbing layers 5b as arrow P2 indicates. This causes black stripes to be observed by the viewer, and thus degrades the viewability of the liquid crystal display device 1.
An object of the present invention is to provide a backlight and a liquid crystal display device that permit the viewing of a displayed image within a desired range of viewing angles with reduced degradation of viewability.
To achieve the above object, according to one aspect of the present invention, a backlight is provided with: a light source; a flat-plate-shaped light guide plate for guiding the light emitted from the light source in a predetermined direction; a light shield louver, disposed so as to face the light guide plate, for shielding part of the light emerging from the light guide plate according to angles of incidence; and a converter for converting the brightness distribution of the light incident on the light shield louver into a predetermined brightness distribution. Here, the brightness distribution of the light incident on the light shield louver is converted in such a way that the brightness at a predetermined angle of incidence within the range from 0xc2x0 to +90xc2x0 and the brightness at a predetermined angle of incidence within the range from 0xc2x0 to xe2x88x9290xc2x0 are higher than the brightness at an angle of incidence of 0xc2x0.
In this configuration, the light emitted from the light source is guided to the light shield louver by the light guide plate. The light incident on the light shield louver is converted so as to have a predetermined brightness distribution by the converter before or after the light emerges from the light guide plate. The light thus converted exhibits the highest brightness in both the positive and negative directions relative to an angle of incidence of 0xc2x0, at which the brightness is lower than the highest brightness.
In the backlight configured as described above, the converter may be composed of prisms arranged at predetermined intervals.
According to another aspect of the present invention, a backlight is provided with: a light source; a flat-plate-shaped light guide plate for guiding the light emitted from the light source in a predetermined direction; a light shield louver, disposed so as to face the light guide plate, for shielding part of the light emerging from the light guide plate according to angles of incidence; and a converter for converting the brightness distribution of the light incident on the light shield louver into a predetermined brightness distribution. Here, the converter shifts the average direction of incidence of the light incident on the light shield louver from the direction normal to the light shield louver.
In this configuration, the light emitted from the light source is guided to the light shield louver by the light guide plate. The light incident on the light shield louver is converted so as to have a predetermined brightness distribution by the converter before or after the light emerges from the light guide plate. The light is incident on the light shield louver from a direction shifted from the direction normal to the light shield louver. Here, the average direction of incidence means the direction indicated by the average of angles at which light is incident on the light shield louver.
In the backlight configured as described above, the converter may have a Fresnel sheet having a sawtooth-shaped section.
In the backlight configured as described above, the brightness distribution of the light incident on the light shield louver may be converted in such a way that the brightness at a predetermined angle of incidence in the positive direction relative to the average angle of incidence and the brightness at a predetermined angle of incidence in the negative direction relative to the average angle of incidence are higher than the brightness at the average angle of incidence.
In this configuration, the light incident on the light shield louver exhibits the highest brightness in both the positive and negative directions relative to the average angle of incidence, at which the brightness is lower than the highest brightness. Here, the average angle of incidence means the average of angles at which light is incident on the light shield louver.
In the backlight configured as described above, the light source may emit varying amounts of light according to directions of emergence. This configuration permits the amount of light emerging from the backlight outside the desired range of angles to be reduced, and thus helps save electric power.
According to another aspect of the present invention, a liquid crystal display device is provided with: a backlight including a light source, a flat-plate-shaped light guide plate for guiding the light emitted from the light source in a predetermined direction, a light shield louver, disposed so as to face the light guide plate, for shielding part of the light emerging from the light guide plate according to angles of incidence, and a converter for converting the brightness distribution of the light incident on the light shield louver into a predetermined brightness distribution; and a liquid crystal panel that displays an image by transmitting the light emerging from the backlight. Here, the brightness distribution of the light incident on the light shield louver is converted in such a way that the brightness at a predetermined angle of incidence within the range from 0xc2x0 to +90xc2x0 and the brightness at a predetermined angle of incidence within the range from 0xc2x0 to xe2x88x9290xc2x0 are higher than the brightness at an angle of incidence of 0xc2x0.
According to another aspect of the present invention, a liquid crystal display device is provided with: a backlight including a light source, a flat-plate-shaped light guide plate for guiding the light emitted from the light source in a predetermined direction, a light shield louver, disposed so as to face the light guide plate, for shielding part of the light emerging from the light guide plate according to angles of incidence, and a converter for converting the brightness distribution of the light incident on the light shield louver into a predetermined brightness distribution; and a liquid crystal panel that displays an image by transmitting the light emerging from the backlight. Here, the converter shifts the average direction of incidence of the light incident on the light shield louver from the direction normal to the light shield louver.
According to another aspect of the present invention, a liquid crystal display device is provided with: a backlight including a light source, a flat-plate-shaped light guide plate for guiding the light emitted from the light source in a predetermined direction, a light shield louver, disposed so as to face the light guide plate, for shielding part of the light emerging from the light guide plate according to angles of incidence, and a converter for converting the brightness distribution of the light incident on the light shield louver into a predetermined brightness distribution; and a liquid crystal panel that displays an image by transmitting the light emerging from the backlight. Here, the converter performs conversion in such a way that the light emerging from the light shield louver exhibits a brightness distribution such that, assuming that the brightness at an angle of emergence of 0xc2x0 is X and the absolute value of the angle of emergence at which the brightness is 0.1X is xcex1, the brightness at an angle of incidence of which the absolute value is xcex1/2 is 0.55X or higher.
In this configuration, the light emitted from the light source is guided to the light shield louver by the light guide plate. The light incident on the light shield louver is converted so as to have a predetermined brightness distribution by the converter before or after the light emerges from the light guide plate. The transmittance of the light shield louver is highest at an angle of incidence of 0xc2x0, and linearly decreases as the angle of incidence increases until it becomes substantially 0% at the limits of the range of viewing angles. The light emerging from the light shield louver, of which the angle of emergence is controlled in this way, is incident on the liquid crystal panel. Here, if the effective range of viewing angles is defined as the range in which brightness is 10% or more of the brightness at an angle of emergence of 0xc2x0 with respect to the light shield louver, within a range of angles that corresponds to half the effective range of viewing angles, brightness is amplified by the converter so as to be 55% or more of the brightness at an angle of emergence of 0xc2x0.